Volume repeatedly deficient? That's a mistake (6)
I believe the answer is:
booboo
'a mistake' is the definition.
(I've seen this before)
'volume repeatedly deficient?' is the wordplay.
'volume repeatedly' can mean two replacements for 'volume'.
'volume' becomes 'boo' (I can't justify this - if you can you should believe this answer much more).
'volume' becomes 'book' (book is a kind of volume).
'deficient?' means to remove the last letter (I've seen 'deficient' mean this).
'book' with its last letter taken off is 'boo'.
'boo'+'boo'='BOOBOO'
'that's' is the link.
Can you help me to learn more?
(Other definitions for booboo that I've seen before include "Having got animated, bear" , "Error" , "Blunder" , "mistake" .)